The study we propose is designed to determine whether blood lead levels which are currently considered "safe" are associated with developmental dysfunction in preschool children. Since 1979, we have been conducting a prospective study of a sample of 249 children selected at birth on the basis of their umbilical cord blood lead level: (low: x=1.8ug/d1, N=85; mid: x=6.5 ug/d1, N=88; high: x=14.6, N=76). We have compiled a comprehensive record of the developmental status of these children at ages 1, 6, 12, 18, and 24 months. By measuring their blood lead and free erythrocyte protoporphyrin levels at 6, 12, 18, and 24 months, we have also compiled a detailed history of the children's lead exposure. The present proposal requests funds to evaluate the developmental status and lead burden of these children at 4 years of age. By integrating these data with those earlier ages, we will be able to address questions inaccessible to cross-sectional studies, namely those pertaining to the interaction among lead dose, timing and duration of exposure, the social/familial context, and developmental outcome. Based on the literature, we will limit our investigation to three aspects of children's functioning: attention (e.g., vigilance, match-to-sample, exploration, incidental learning), language (e.g., morphology, syntax, verbal associative memory, auditory comprehension, word finding), and fine motor function (e.g., finger dexterity, arm-hand steadiness, control precision). In recognition of the difficulties which confounding poses to epidemiological studies of this type, we will gather data on five classes of possible confounding variables: child (e.g., pica, temperament, serum ferritin, blood cadmium levels), parental (e.g., IQ, recent life change), family (e.g., SES, family structure), socialization values and behavior (e.g., child rearing practices, quality of rearing environment) and extra-family contacts and supports (e.g., child's day care history, social support systems). The specific issues which we will investigate include (1) the existence of sensitive or critical periods with regard to lead toxicity, (2) the cognitive functions most vulnerable, and (3) the environmental factors which increase or reduce a child's risk of lead-induced deficit.